1. Field of the Invention
The present invention relates to a novel method of preparing a vinyl chloride resin plastisol easily. More particularly, it relates to a method of preparing a plastisol merely by the act of immersing aggregates of vinyl chloride resin particles adapted for paste processing in a plasticizer.
2. Description of the Prior Art
For the paste processing of a vinyl chloride resin, a fine particulate vinyl chloride resin prepared for the paste processing is mixed with a plasticizer and a stabilizer and, when necessary, with pigments, fillers and other compounding ingredients to prepare a liquid plastisol and the plastisol thus prepared is used for molding such as casting, coating dipping and the like.
In the technique of paste processing of vinyl chloride resins, it is very important for providing molded articles having excellent appearance with delicate fine surface structures and prevent decreased mechanical strengths in the molded articles, that the major part of the vinyl chloride resin is dispersed in the plasticizer in a condition such that the particles are divided almost to the size of the primary particles.
It is generally required that the vinyl chloride resin used for paste processing is in the form of fine particles of spherical shape having the primary diameter of 0.1 to 5 .mu.m. The vinyl chloride resin for the paste processing is generally prepared by preparing a polymer latex by emulsion polymerization or by microsuspension polymerization, then, making from the polymer thus prepared a powder containing aggregates of the primary particles in spray drops by spray drying and, finally, pulverizing the aggregates by a pulverizer.
For preparation of the paste using the vinyl chloride resin thus prepared, the vinyl chloride resin particles are mixed with a plasticizer in a mixer, such as a crushing mixer, a Henshel mixer, a biaxial mixer and the like. In the mixing, it is required that the major part of the resin particles is divided to the size of the primary particles, or that, if this condition is difficult to achieve, the aggregates obtained after the pulverizing described above are not further aggregated to form larger particles.
However, the individual particles in the aggregates formed by spray drying of the latex are bound together firmly and are not easily pulverized to the size of the primary particles by the pulverizing operation by a pulverizer. Thus, the powder produced after the pulverizing actually is a mixture containing fragments of the aggregates having various sizes.
Two things must be considered for the dispersion of the resin particles in the preparation of a plastisol. The first of them is that the aggregates must be dispersed by decomposing at least to the size of the pulverized particles described above because the aggregates of the resin particles behave as a combined block of particles in the plasticizer. The second of them is that larger fragments of the aggregates still remaining when the major part is dispersed by decomposing to the size of the pulverized particles must be demolished for dispersion by the shearing force of a mixing step.
When these things are considered, an onerous process is conventionally required for achieving good dispersion of particles which employs a powerful mixing apparatus and only a limited amount of plasticizer is used for dispersion at first so that the shearing force of the mixer works efficiently on a rather solid mixture before the remaining plasticizer is added.
In conventional methods, the size of the droplets in the drying process is made smaller for more efficient drying of the powder and for adapting to the application of the paste described above. Thus, particles of the average diameter of 60 .mu.m or less are formed in the drying process and the dried particles are then pulverized by a pulverizer to produce particles having an average diameter of 10 to 20 .mu.m.
In plastisols prepared using particles without the pulverizing process or prepared by merely immersing the particles pulverized by the pulverizing process in the plasticizer, the particles behave like combined blocks of many particles having diameters of 1 mm or more. In these plastisols, the plasticizer does not penetrate into the inside of the blocks of the particles even though the outer surface of the blocks are wet with the plasticizer. Plastisols having a heterogeneous condition of this kind cannot be advantageously utilized for various kinds of processing.